Changes in the Protein Secondary Structure on the Surface of Silica Nanoparticles with Different Sizes

Nanoparticles (NPs) are highly promising for medical applications; however, their toxicity is a limiting factor. Understanding the interactions between NPs and proteins is crucial for mitigating toxicity concerns and advancing the safe use of NPs in the biomedical field. Important factors governing NPs-protein interactions include the size (curvature), surface charge, and surface state of NPs as well as coexisting ions in solvents. In this study, we focused on the effect of the NP size (curvature) on the protein secondary structure using silica NPs (SiNPs) with diameters of 10 nm, 100 nm, 1 μm, and 10 μm. The secondary structure of bovine serum albumin (BSA) that interacted with SiNPs was analyzed via thioflavin T (ThT) fluorescence, Fourier transform infrared spectroscopy (FT-IR), and circular dichroism (CD). Furthermore, the stirring time was varied to 1, 24, and 48 h, and the effect of the incubation time was investigated. ThT measurements showed that the β-sheet ratio of BSA was the highest when incubated with SiNPs of 10 nm diameter for 1 h. This result can be attributed to the characteristics of small SiNPs such as high curvature and large surface area per mass, facilitating more extensive interactions with BSA. Interestingly, the dependence of the ThT fluorescence intensity on the NP diameter did not show a linear pattern. This is potentially caused by a complex interplay of factors including changes in the curvature and the total surface area of SiNPs. Notably, ultrasmall SiNPs exhibited the potential to induce an abnormal protein conformation. The relationship between the SiNP size and protein secondary structure change presented in this study sheds light on critical factors for the safe and effective application of NPs in future biomedical applications.